JP3135644B2 - Reactor core - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a nuclear reactor core,
In particular, it relates to a reactor core loaded with a plutonium-uranium mixed oxide (MOX) fuel assembly.

[0002]

2. Description of the Related Art FIG. 7 shows the structure of a fuel assembly constituting a boiling water reactor core. 4 is an upper tie plate, 5 is a channel fastener, 13 is a channel box, 8 is a spacer, 9 is a lower tie plate, and 14 is a fuel rod.
Further, pellets obtained by sintering uranium dioxide as a nuclear fuel substance are sealed in the fuel rods 14.

As shown in FIG. 8, the boiling water reactor core, called a D-shaped lattice, has a different gap interval between adjacent fuel assemblies, and the control rod 11 is inserted on the control rod side (control rod side). 8) (D ₁ in FIG. 8, hereinafter referred to as a wide gap) is the gap interval (D ₂ in FIG. 8, hereinafter referred to as a narrow gap) on the side where the control rod 11 is not inserted (the side without the control rod). ) Wider. Therefore, on the wide gap side, the moderator passage area outside the channel box 13 is large, and the neutron moderating effect is large. For this reason, the output of each fuel rod 14 in the fuel assembly 12 tends to be high on the wide gap side and low on the narrow gap side.

[0004] In order to secure the thermal margin of the fuel, it is desirable to flatten the fuel rod output distribution (referred to as local power distribution) of the fuel assembly. For this reason, in the fuel assembly loaded in the D-shaped lattice core described above, the level of the local power distribution based on the difference in the gap width is adjusted by increasing or decreasing the uranium enrichment in the fuel rod. That is, a fuel rod with low enrichment is used on the wide gap side where the output tends to be high, and a fuel rod with high enrichment is used on the narrow gap side where the output is low. For this reason, there are many types of fuel rods and types of enrichment.

Further, in the boiling water reactor, as a method of correcting the axial power distribution due to the void distribution in the core axial direction, a technology using an enrichment difference in the axial direction described in Japanese Patent No. 1286588 is adopted. Therefore, a plurality of uranium enriched pellets are used in one fuel rod.

[0006]

The enrichment distribution technique described above is a well-established technique for uranium fuel.

However, a mixed oxide using plutonium as a fissile material (Mixed Oxide, hereinafter referred to as MOX)
In the case of fuel, since the molding of fuel pellets is performed in a completely sealed container (glove box), the unit price of molding is higher than that of uranium fuel. The number increases. This increases the downtime of the processing equipment, which causes an increase in molding cost. Therefore MOX
For fuels, it is necessary to simplify the enrichment distribution as much as possible and to reduce the types of plutonium enrichment in order to keep the molding processing cost low. Further, if the enrichment design is simplified, it is advantageous to improve the accuracy of nuclear characteristic analysis in MOX fuels with a small track record.

Further, the MOX fuel assembly requires a molding cost,
Due to the high transportation cost of uranium fuel, it is necessary to consider reducing the number of processed and transported vehicles. By increasing the amount of plutonium loaded per fuel assembly, plutonium can be concentrated in a small number of fuel assemblies. It is better to use.

An object of the present invention is to provide a nuclear reactor core capable of increasing the amount of plutonium loaded per MOX fuel assembly and simplifying the enrichment distribution.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a fuel assembly in which the number of cores loaded in the first fuel assembly is less than that of the second fuel assembly, and the average burn-out of the first fuel assembly is reduced. The number of types of fuel rods constituting the first fuel assembly is smaller than the number of types of fuel rods constituting the first fuel assembly, which is smaller than the average burnup of the second fuel assembly , Primary fuel
Aggregates are plutonium and fissile as early fissile material
MOX fuel assembly containing uranium and second fuel assembly
Pulls uranium as fissile material in early combustion
This can be achieved by a uranium fuel assembly containing no tonium .

[0011]

According to the above features, the types of fuel rods of the MOX fuel body can be made smaller than those of the uranium fuel assembly, and the types of plutonium enrichment can be reduced.

Further, by making the axial enrichment distribution of the MOX fuel assembly uniform, the enrichment distribution can be simplified.

As a result, the man-hours for molding the MOX fuel assembly can be reduced, and the molding cost can be reduced.

[0014]

[Example] In the plutonium utilization plan in a light water reactor (pull thermal plan), a uranium fuel assembly is taken out at the end of a cycle of a core loaded with a uranium fuel assembly, and a MOX fuel assembly is loaded as a new fuel instead. It is planned that the uranium fuel assemblies will be sequentially replaced with MOX fuel assemblies, and the proportion of MOX fuel assemblies will be increased in stages. This is because it is desirable to sequentially confirm the core characteristics and the operation characteristics by gradually increasing the loading rate of the MOX fuel assembly.

[0015] In the practical scale demonstration of the planned pluthermal project, MOX fuel assemblies will be loaded to 1/4 of the core at the final scale.

Since the MOX fuel assembly has a low irradiation performance, the MOX fuel assembly starts with a lower burnup target than that of the uranium fuel assembly and gradually increases.

In the present embodiment, the feature of the loading rate of the MOX fuel assembly to the core in the light water reactor in a stepwise manner is increased, that is, (i) the MOX fuel assembly is more effective than the uranium fuel assembly. (Ii) the MOX fuel assembly has lower burnup than the uranium fuel assembly, and the bundle output is lower in the core; (iii) the uranium fuel assembly in the core has higher and lower enrichment Focusing on the difference, in order to achieve the above-mentioned object, the following improvements were made in the present embodiment.

(1) Reduction of MOX fuel rod types (1) The MOX fuel assembly used in the practical scale demonstration has a lower burnup / enrichment (enrichment) than the uranium fuel assembly, and has an infinite multiplication factor. Since the change is gradual, the gadolinia design has a characteristic that it is less likely to become a hot channel than in the same uranium fuel assembly in a mixed core of the uranium fuel assembly and the MOX fuel assembly.

That is, if the bundle output is allowed to increase to the same extent as the uranium fuel assembly, the number of gadolinia rods in the MOX fuel assembly can be reduced.

Since gadolinia is a strong neutron absorber, the output of gadolinia rods is lower than that of ordinary uranium rods and MOX rods. Therefore, as the number of gadolinia rods increases, the output peaking of ordinary uranium rods and MOX rods increases. Get higher.

For this reason, as the number of gadolinia bars is reduced, local output peaking in the aggregate can be reduced. This leads to a reduction in the number of MOX fuel rod types.

(2) Uniform distribution of enrichment in the axial direction Since the loading ratio of the MOX fuel assemblies in the replacement batch is small, the MOX fuel assemblies are distributed by the effect of the vertical enrichment distribution of the surrounding uranium fuel assemblies. In the body, the thermal limit can be satisfied even without the axial enrichment distribution.

The MOX fuel body can be arranged at a low power portion in the reactor core by devising the fuel loading position.

Therefore, in the MOX fuel assembly, the axial enrichment distribution is made uniform, and the enrichment distribution design is simplified.

An embodiment of the present invention will be described below with reference to FIGS.

FIG. 1 is a diagram showing a horizontal sectional view of a boiling water reactor core according to the present invention. The reactor core includes two types of fuel assemblies, namely, a uranium fuel assembly (second fuel assembly). ) 20 and a MOX fuel body (first fuel assembly) 21 are regularly arranged. Numbers 1 to 4 shown in the fuel assembly of FIG. 1 indicate the number of cycles in which the fuel assembly stays in the furnace, 1 is a new fuel assembly (or first cycle fuel),
Reference numeral 2 denotes a second cycle fuel assembly, 3 denotes a third cycle fuel assembly, and 4 denotes a fourth cycle fuel assembly. Both the uranium fuel assembly 20 and the MOX fuel assembly 21 have the structure shown in FIG.

In this reactor core, out of the total number of 548 fuel assemblies, 156 MOX fuel assemblies, 392 uranium fuel assemblies, and a smaller number of MOX fuel assemblies,
Among the 152 new fuel assemblies (first cycle fuel assemblies numbered 1 in FIG. 1), the number of MOX fuel assemblies is 52, which is smaller than the number of 100 uranium fuel assemblies.

The number of cycles in which the MOX fuel assembly stays in the core is three, while the number of cycles in the uranium fuel assembly is four, and the average number of cycles in the uranium fuel assembly is longer in the reactor. Therefore, the removal burn-up is higher for the uranium fuel assembly than for the MOX fuel assembly.

Further, the present core has a control cell comprising four fuel assemblies and control rods for adjusting the power of the reactor. The fuel loading method is described in Japanese Patent Application No. 2-225079. Also, a technique is employed in which a MOX fuel assembly is arranged in a first region of the second layer from the outermost periphery and in a second region adjacent to the control cell to increase the thermal margin of the MOX fuel assembly.

FIG. 2 shows the arrangement of gadolinia fuel rods 26 in the first fuel assembly (uranium fuel assembly) 20 of the two fuel assemblies constituting the nuclear reactor. Also,
FIG. 3 shows an arrangement of gadolinia fuel rods 27 in the second fuel assembly (MOX fuel assembly) 21. In general, it is said that the MOX fuel assembly requires more gadolinia rods than the uranium fuel assembly because the absorption cross-sectional area of plutonium is larger than that of uranium. The present embodiment is characterized in that the MOX fuel assembly has a smaller number of gadolinia than the uranium fuel assembly. This is because when comparing the infinite multiplication factor combustion changes of the uranium fuel assembly and the MOX fuel assembly shown in FIG. 9, (1) M
Since the OX fuel assembly has less loss of fissile material than the uranium fuel assembly, the slope of infinite multiplication factor (combustion change) is gentle, and its maximum value is smaller than that of the uranium fuel assembly. .

(2) Since the MOX fuel assembly of the present reactor has a smaller extraction burnup than the uranium fuel assembly, the infinite multiplication factor is smaller than that of the uranium fuel assembly.

It can be seen that: Therefore, in a core in which these two fuel assemblies are mixed, the MOX fuel assembly has a characteristic that it is less likely to become a hot channel than a uranium fuel assembly having the same gadolinia design. Therefore, if the bundle output is allowed to increase to the same extent as the uranium fuel assembly, the number of gadolinia rods in the MOX fuel assembly can be reduced. As a result, in the present embodiment, the number of gadolinia rods in the uranium fuel assembly is 12, whereas that in the MOX fuel assembly is 8.

Since gadolinia is a strong neutron absorber, the output of gadolinia rods is lower than that of ordinary uranium rods and MOX rods.
There is a characteristic that the output peaking of ordinary uranium fuel rods and MOX fuel rods is increased.

Therefore, the local output peaking in the aggregate can be reduced as the number of gadolinia bars is reduced. This leads to a reduction in MOX fuel rod types.

FIG. 4 is a diagram showing the types of fuel rods of the uranium fuel assembly 20. Twelve types of fuel rods 41 to 52 having different enrichment distributions are required. On the other hand, in the MOX fuel assembly shown in FIG. 5, there are only six types of fuel rods having different enrichment distributions from 61 to 66, and
This contributes to the simplification of the design of the enrichment distribution of the MOX fuel assembly, which is the object of the present invention.

Further, in the MOX fuel assembly, gadolinia combustion is slower than that of the uranium fuel assembly, so that it is not necessary to use various types of gadolinia concentration as in the uranium fuel assembly. As a result, as shown in FIG. 2 is a uranium fuel assemblies with respect to the G ₁ was required a three gadolinia rods up to G _3, in the case of MOX fuel assembly,
As shown by G in FIG. 3, there is only one type, which can be simplified.

Further, in the MOX fuel assembly shown in FIG. 3, in order to make the local output peaking particularly flat, the cross section of the fuel assembly described in Japanese Patent Application No. 2-225079 is formed by the channel fastener. When divided into a first area including a corner where the channel fastener is located and a second area not including a corner where the channel fastener is located, with a diagonal line connecting two opposite corners that are not located as boundaries, A fuel assembly, wherein the average enrichment of plutonium in one region is lower than that in the second region, and the number of fuel rods containing the burnable poison is greater in the first region than in the second region. . By doing so, the types of MOX fuel rods are further reduced. Next, as described with reference to FIG. 1, the number of MOX fuel assemblies in the replacement batch is smaller than the number of uranium fuel assemblies, so that the uranium fuel assemblies that occupy most of the core are enriched in the axial direction. If the power difference is increased, the power distribution of the core can be flattened. In addition, the MOX fuel assembly is arranged adjacent to the control cell and in the core low power portion of the second layer from the outermost periphery. Therefore, the MOX fuel assembly can satisfy the thermal limit value even without the axial enrichment distribution. Therefore, as shown in FIG. 6, since the uranium fuel assembly 20 occupying most of the core has an axial enrichment,
The MOX fuel assembly 21 can make the axial enrichment distribution uniform.

FIG. 10 shows a specific enrichment distribution of the MOX fuel assembly shown in FIG. There are six types of fuel rods,
One of these is a gadolinia rod. Of the remaining five fuel rods, the control rod side (opposite the channel fastener)
The two types of fuel rods in, as uranium rods, reduce the number of MOX fuel rods and have a uniform axial enrichment distribution.
The enrichment type can be made into three types, and the gadolinia rod does not contain MOX but only uranium, but it was used to reduce the number and also to reduce the MOX enrichment type. Since the number of uranium rods at the corners is also small, the distribution of enrichment can be made simple without significantly reducing the amount of plutonium loaded per unit, and plutonium can be concentrated in a small number of fuel assemblies. It can be used, and the number of processed and transported bodies of the MOX fuel assembly can be reduced.

In the example described here, the first fuel assembly is a MOX fuel assembly. However, other special fuel assemblies may be used. For example,
Uranium absorbed by reprocessing (called recovered uranium)
Is an example.

The recovered uranium contains uranium isotopes such as ²³⁶ U which are not present in natural uranium, but its daughter nuclide becomes a high-energy γ-ray source, so that the exposure dose during processing slightly increases. It is known that reduction of the number of processed bodies and simplification of the pellet enrichment distribution are as important issues as in the case of the MOX fuel assembly.

In this case, the recovered uranium fuel assembly is limited to a small amount in the core, and the burn-up is made smaller than that of the uranium fuel assembly to be mixedly loaded. It is also possible to simplify the enrichment distribution.

[0042]

According to the present invention, the enrichment distribution of MOX fuel assemblies can be simplified in a nuclear reactor in which a smaller number of MOX fuel assemblies than uranium fuel assemblies are loaded in the core.

[Brief description of the drawings]

FIG. 1 is a plan view of a core of a nuclear reactor according to a preferred embodiment of the present invention.

FIG. 2 is a cross-sectional view of the uranium fuel assembly of FIG.

FIG. 3 is a cross-sectional view of the MOX fuel assembly of FIG.

FIG. 4 is an explanatory view showing a fuel rod arrangement of the uranium fuel assembly of FIG. 2;

FIG. 5 is an explanatory view showing a fuel rod arrangement of the MOX fuel assembly of FIG. 3;

FIG. 6 is an explanatory diagram showing the axial distribution of fissile material in each of the fuel assemblies of FIGS. 2 and 3;

FIG. 7 is an external view of a fuel assembly.

FIG. 8 is an explanatory diagram of a D lattice core.

FIG. 9 is a characteristic diagram showing changes in the infinite multiplication factor of the uranium fuel assembly and the MOX fuel assembly.

FIG. 10 is a characteristic diagram showing the enrichment and enrichment distribution of each fuel rod of the MOX fuel assembly of FIG.

[Explanation of symbols]

4: Upper tie plate, 5: Channel fastener, 8 ...
Spacer, 9: Lower tie plate, 13: Channel box, 11: Control rod, 12: D-shaped lattice, 14: Fuel rod, 20: Uranium fuel assembly, 21: MOX fuel assembly, 22: Upper and lower ends of fuel assembly Part of natural uranium part, 23 ...
High enrichment part at the upper part of the fuel assembly, 24 ... Low enrichment part at the lower part of the fuel assembly, 25 ... Low enrichment part at the top of the fuel assembly, 26
... Gadolinia rods for uranium fuel assemblies, 27 ... Gadolinia rods for MOX fuel assemblies, 41-52 ... Fuel rods for uranium fuel assemblies, 61-66 ... Fuel rods for MOX fuel assemblies.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Nobuo Sekine, 3-2-1 Sachimachi, Hitachi, Ibaraki Prefecture Inside Hitachi Engineering Co., Ltd. (72) Akira Nishimura 3-1-1, Sachimachi, Hitachi, Ibaraki Hitachi, Ltd. Hitachi Plant (56) References JP-A-58-178286 (JP, A) JP-A-3-251794 (JP, A) JP-A-3-267795 (JP, A) (58) Field (Int.Cl. ⁷ , DB name) G21C 5/18 G21C 5/00 G21C 5/12

Claims (3)

(57) [Claims] 1. A reactor core in which at least two types of fuel assemblies are simultaneously loaded, wherein the number of core loadings of the first fuel assembly is less than the number of core loadings of the second fuel assembly; The average burnup of the first fuel assembly is smaller than the average burnup of the second fuel assembly, and the number of fuel rods constituting the first fuel assembly is equal to the second fuel assembly. Less than the number of fuel rod types constituting the first fuel
Aggregates are plutonium and fissile as early fissile material
A MOX fuel assembly comprising uranium and uranium;
Coalescence contains uranium as fissile material in the early stage of combustion
A reactor core for a uranium fuel assembly that does not contain plutonium . 2. A reactor core in which at least two types of fuel assemblies are simultaneously loaded, wherein the number of core loadings of the first fuel assembly is less than the number of core loadings of the second fuel assembly; and average discharge burnup of the first fuel assembly, on which is smaller than the average discharge burnup of the second fuel assembly, a second fuel assembly has an axial enrichment difference, first A fuel assembly having a uniform enrichment distribution and an enrichment distribution in an axial direction. 3. A reactor core in which at least two types of fuel assemblies are simultaneously loaded, wherein the number of core loadings of the first fuel assembly is less than the number of core loadings of the second fuel assembly, In addition, the number of gadolinia-containing fuel rods of the first fuel assembly is smaller than the number of gadolinia-containing fuel rods of the second fuel assembly , and the first fuel assembly is fissile in the early stage of combustion.
MOX fuel collection containing plutonium and uranium as substances
In the coalescence, the second fuel assembly has a nuclear
Uranium containing uranium as a fission substance and not containing plutonium
A nuclear reactor core characterized by being a fuel assembly .